1. Field of the Invention
The present invention relates to a system for monitoring the conditions of a fine wire bonded interconnection and more particularly to a monitoring system for incorporation into a high-speed, automatic wire bonder so that continuous operator attention is not required on each automatic wire bonder.
2. Description of the Prior Art
Automatic wire bonders are known in the semiconductor manufacturing industry. A commercially available processor controlled automatic wire bonder is made by Kulicke and Soffa Industries, Inc. and is shown and described in U.S. Pat. No. 4,266,710. The wedge bonding mechanism for an automatic wedge wire bonder also commercially available from Kulicke and Soffa Industries, Inc. is shown and described in U.S. Pat. No. 4,239,144.
Heretofore, it was common practice to assign an attendant to one or more automatic wire bonders. These high-speed wire bonders complete an interconnection of fine wire between a first and second bond position in approximately 250 milliseconds. If the fine wire breaks and/or is not properly fed from the wire feed to the bonding tool, a tail of proper length is not positioned below the working face of the bonding tool to permit a proper bonding operation on the next bond. Numerous problems occur which can cause a wire to be missing from the working face of a bonding tool. In addition, other problems occur which cause the first or second bond to be made improperly or to become disconnected from the pad or terminal on which it is made. If the attendant responsible for an automatic wire bonder is responsible for detecting the numerous errors which can and do occur, a large number of semiconductor devices could be processed or operated on by the automatic wire bonder before the attendant could shut the machine down. After making the first error, the bonding tool can continue to attempt to make bonds at the first and second bond position without making a good wire interconnection. If the ball or tail is missing from the end of the wire under the working face of a bonding tool when the bonder attempts to make a subsequent first and second bond, the bonding tool can crash into the terminal pads on the semiconductor device and/or destroy the lead out pads especially when ultrasonic scrubbing is employed to speed up the bonding operation. Ultrasonic scrubbing of pads and terminals with a bare bonding tool will damage the electrodes.
Attempts have been made to monitor the condition of a bond at the time it is being made by an ultrasonically driven bonding tool. The prior art devices have monitored the drive current as well as the impedance of the bonding transducer to determine if the bonds being made by the bonding tool are properly attached to the terminals or pads. Through a complex analysis of the changes in impedance relative to the bonding time such prior art systems have been able to determine with some accuracy whether the first and/or second bond was properly made. However, such prior art bond monitors did not monitor whether the first bond subsequently became detached or whether the fine wire interconnection between the first and second bond broke or was made improperly. U.S. Pat. Nos. 4,341,574 and 3,852,999 are typical of systems which measure the impedance of the transducer to determine if the bonds are properly attached to the terminals or pads.
Heretofore the conductance of a fine wire interconnection has been measured during a complete bonding operation. However, heretofore the systems employed for monitoring a bonding operation have employed a DC voltage source which has been applied to the fine wire. The prior art systems included some means for measuring current changes in the fine wire. This required that the fine wire be insulated or isolated at the wire feed and that the fine wire be grounded at the semiconductor or the work station. The bonding tool and wire feed were insulated so that the current path from the voltage source was directed through the fine wire to the pad or electrode on the device to ground. Some devices required that the conductive path be reversed so that current would flow through the semiconductor device being tested. If the voltage source of the prior art conductive devices could be made stable, changes in the current observed were proportional to the impedance of the fine wire plus the impedance of the device being bonded which includes capacitive and resistive components. These prior art systems would ground the fine wire by making contact to the semiconductor device and providing a path to ground. In these prior art systems the first bond and the second bond must be grounded and problems that occur in sensing the interconnection after the first bond become extremely difficult with conductive impedance measuring instruments or current sensing devices. Attempts to raise the voltage or current in the fine wire to generate larger current flow so as to provide larger values for detection quite often become harmful to the device and can cause destruction of the more sensitive devices. Since some electrodes or pads on the devices are more sensitive than others, it was necessary to shut off or disconnect the monitoring system to prevent damage to some devices. The gates of field effect transistors (FETs) have very high impedance and can be easily destroyed with small amounts of current. Other devices having high impedance cannot be properly tested due to very small current changes induced into the prior art conductive monitoring systems. CMOS devices have a very small input capacitance and are easily destroyed if current and voltage sources are raised too high. Other types of devices cannot be tested without reversing the polarity of the current flow. When the device under test presents a high capacitive time constant, the device can continue to charge after the first bond is made and create false indications of bad bonds due to decaying current flow. Any speed-up of detection before the device and circuit is fully charged is a compromise which could easily fail to detect some types of improper fine wire interconnections.
Applicants concluded it would be extremely desirable to provide a wire bond monitoring system that would continue to monitor a fine wire interconnection being made between a first and second band without diminishing its sensitivity in any way. Such a monitor would be commercially desirable if it would work on all types of semiconductor devices without having to change the voltage source or current source settings for different types of devices and would not have to avoid certain terminals on the semiconductor device or to employ manually selectable switches to provide reverse polarity. Such a wire bond monitor would be extremely desirable if it was inexpensive and simplified and structured so as to be incorporated into existing automatic wire bonders without requiring modifications which would change the mode of operation on automatic wire bonders. Further, it would be desirable if the wire bond monitor would provide a miniaturized structure which could be easily incorporated into the existing automatic wire bonders and would not be affected by thermal changes and vibrations of the bonding transducer or rapid movement of the bonding head.